1. Field of the Invention
This invention relates to a CCD (charge coupled device) imager in which signal charges are transferred by a plurality of juxtaposed read-out registers.
2. Description of the Prior Art
In a CCD imager, such as an interline transfer (IT) or frame interline transfer (FIT) type CCD imager, there is known a structure, such as from the Japanese Patent Laid Open Publication No. 59-13369 (1984), in which, for reducing the read-out frequency of the horizontal register, signal readout is performed by a plurality of horizontal registers.
FIG. 1 shows an example of a dual read-out FIT type CCD imager in which signal read-out is performed by two horizontal registers. Signal charges are produced by photoelectric conversion in an image area 101 and transferred via a storage area 102 to a first horizontal register 103 and a second horizontal register 104. This storage area 102 is made up of a plurality of vertical registers and signal charges of every second vertical register are allocated to the first horizontal register 103 and the second horizontal register 104. It is noted that the signal charges to be transferred to the second horizontal register 104 at the distant side from the image area 101 are transferred by way of the first horizontal register 103.
However, the above described CCD imager is beset with the problem that the transfer efficiency between the horizontal registers can not be improved.
That is, the transfer electrodes of the horizontal registers 103, 104 are arranged, in view of the transfer along the horizontal direction, at a predetermined pitch along the horizontal direction. However, the vertical length of the transfer electrode of the horizontal register becomes longer than the horizontal length, such that, with an increase in the vertical length l.sub.1 of the electrode 105, as shown in FIG. 2, changes in the potential along the vertical direction are correspondingly reduced. The result is that flat potential portions are produced to lower the charge transfer efficiency from the first horizontal register 103 to the second horizontal register 104 in the vertical direction.
In order to solve this problem, fine potential steps can be formed, such as by ion implantation, along the vertical direction of the transfer electrode of the horizontal register. However, in such case, the manufacturing process is significantly complicated due to the necessity of forming photo resists and so forth.